system(d): allow auto net configuration with networkd

system(d): allow auto net configuration with networkd

Allow automatic network configuration via systemd-networkd if selected.

If systemd-networkd is enabled and $BR2_SYSTEM_DHCP is set, then create
a .network file to configure the selected network interface via DHCP.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
[Thomas:
 - merge the two patches from Eric into just one
 - instead of generating the dhcp.network file completely from the .mk
   file, use a template file, and "sed" it with the right network
   interface]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>